1. Field of the Invention
The invention relates to printing devices for document processing machines. More particularly, it relates to rotary impact endorsers for such machines and to improvements which assure more favorable load configurations for rotary impact endorsers.
2. Description of the Prior Art
Prior art stamp endorsers employed with document processing machines have included endorse-on-demand mechanisms relying on print heads rotationally impacting against a document currently passing an endorse station, including a platen at rest, as illustrated in FIG. 1. The print head rotates to bring the stamp die to bear against the document and platen; and, having been inked, the die carried by the print head leaves an impression on the document. The print head then continues to rotate, disengaging the platen, past an inking roller until it reaches a home position.
Such prior art systems have proven to be acceptable when print head velocities are low with proportionately low impact torques as in presently used moderate speed systems where document track speed approximates 12 inches per second. Systems employing higher track speeds, of the order of 75 inches per second, have been achieved in the past on early model low speed machines by use of essentially a brute force approach in which a relatively large motor was used to maintain momentum and supply significant torque through print head engagement impact. This brute force approach has proven reliable, but has its disadvantages. The disadvantages include requirements for larger and costlier motors and the existence of higher mechanism bearing loads due to high platen spring preloading.
The stamp endorser for a more recent higher speed machine is required to perform for over 40 million endorse cycles while reaching a track speed on the order of 100 inches per second. The endorser is allotted a space adjacent to the track which dictates either a small motor (about 2 inches in diameter) for directly driving the print head on the motor shaft or a remotely mounted larger motor with a drive system, typically comprising belts or gearing. Small motors predictably generate low torques which have been found to present a problem in such cases, where a large engagement torque has to be overcome while a constant speed is maintained. Larger motors employing a drive system add significantly to cost and complexity in addition to requiring comparatively large, expensive platen roller and pivot bearings and bearing structures as a result of the high platen spring loads needed. Also, if a reasonably low spring rate is to be employed, the spring size increases significantly. In a system to which the present invention has been applied, considering the space allotted as dictated by the document size capabilities specified for the system, difficulties would have been encountered in adapting a platen structure for a "brute force" mechanism.
In addition to the foregoing, the higher speed machines have problems of reliability with the print heads which is also a result of the impact at engagement. The potential exists for the impact against a localized area of the stamp, specifically the leading edge, to peel the stamp away from the print head. This is not a problem on some endorsers where an indexing metal print wheel is located at the print head edge to absorb impact. However, problems of peeling stamps are experienced on moderate speed endorsers in which a rubber stamp edge absorbs the impact. Incidences of peeling may be expected to rise on a similar higher speed endorser with its much higher operating speed.
It becomes apparent then that edge or engagement impact is a great detriment. It has been determined to cause decreases in both performance and reliability in direct proportion to increases in impact forces.
A prior art solution to the problem of edge impact has been to convert the instantaneous edge rise into a gradual rise by incorporating a "ramp" into the stamp leading edge. In effect this "ramp" then is a cam profile and the platen acts as a cam follower. This approach is an improvement and in fact, to this end, a slight chamfer is applied on the current stamp edges. When considerations are made for variations in document thickness, the inherent looseness of bonded rubber stamp print head tolerances and swelling of stamps in the presence of hydrocarbon inks it is found that the rise can be significant. It is especially significant, since it occurs at a time in the cycle when additional torque is needed to accelerate the print head to speed.